1. Field of the Invention
This invention is related to continuously variable transmission systems and more particularly to a continuously variable transmission system having a variable diameter pulley assembly responsive to axially directed forces.
2. Description of the Prior Art
Continuously variable transmissions of the class which may be broadly characterized as a transmission system in which a belt couples a pair of pulleys, at least one of which can assume a more or less continuous range of effective diameters, these types of transmission systems generally fall into two categories: (a) those systems employing V-belts or variations thereof (such as link belts or chains) for transmitting power from one pulley to the other and (b) those systems employing flat, flexible belts between variable diameter pulleys.
Those skilled in the art have come to appreciate that continuously variable transmissions capable of using flat belts have significant fundamental advantages over those systems employing V-belts. In the case of the latter, the belts are composed of various compositions and have a trapezoidal cross section. The belts transmit rotary motion at one speed from a source of power, such as an engine or motor, to an output shaft which rotates at a different speed, the speed ratio between the input speed received from the source and the output speed being varied in a continuous fashion from a minimum to a maximum dependent on the geometry of the belt and the pulley system. The V-belt is compressed between smooth, conical facing surfaces in each of the two pulleys by external axial forces. These axial forces apply a compressive force on the sides of the belts to prevent slippage. In operation, a displacement caused by the change in the axial loading on the conical surfaces causes the V-belt to change its radial positions in the two pulleys until a force balance is achieved or a limit range stop is reached.
For large transmitted torque, the axial forces exerted on the facing conical surfaces produce large compressive forces on the V-belt. This requires that the V-belt have a substantial thickness to prevent it from laterally collapsing. This thickness increases the centrifugal force acting on the belt and causes higher belt tension loads. In addition, as the belt thickness increases, the pulley size must be increased due to higher stress loads at a given minimum pulley radius. Further, the frictional forces acting on the sides of the V-belt tend to hold it to the facing conical surfaces, therefore, the V-belt must be continuously "pulled out" from between the conical surfaces upon leaving each of the pulleys. This results in significant friction losses and belt fatigue which adversely affects the overall efficiency of the V-belt system and its operating life. Consequently, although variable speed V-belt drives are being used in a wide variety of applications they are severely limited in their power transfer capabilities for small size equipment. Further, V-belts are generally not compatible with the serpentine belt systems used in automotive applications because their bending in a reverse direction is severely limited.
Because of these inherent drawbacks to the use of V-belts, the second category has been developed which is broadly designated as flat belt drive continuously variable transmissions. As the name suggests, flat belts may be employed between the driven and driving pulley assemblies which are dynamically variable in diameter to obtain the sought after ratio changes. No axial movement between the two pulley rims is necessary. On the other hand, it is necessary to somehow effect the diametric variations of the individual pulley assemblies. In one particularly effective system, this function is achieved by causing a circular array of drive elements in each pulley to translate radially inwardly or outwardly in concert to obtain a given effective diameter of the pulley assembly. Variable speed flat belt transmissions of this type and their associated control systems are disclosed in U.S. Pat. Nos. 4,295,836, 4,591,351, 4,714,452 and 4,768,996. All of these patents have been issued to Emerson L. Kumm. In all but the first patent enumerated above, the variable diameter pulleys include a pair of pulley rims between which extend a series of belt engaging elements that are simultaneously moved in a radial direction to change the effective diameter of the pulley.
Each pulley assembly includes two rim elements which are made up of two pairs of disks (designated, respectively, the inner disk members and the outer disk members) which are concentric with each other and disposed immediately adjacent one another. Each of the adjacent disk members has a series of spiral grooves or guideways. The guideways of one of the disks are oriented in the opposite sense to the guideways of the other. The ends of the belt engaging elements are captured at the intersections of the spiral guideways of the two adjacent disks. Thus, radial adjustment of the position of the belt engaging elements may be achieved by a relative rotation of displacement between the inner and outer disk members. This rotation being, of course, carried out simultaneously and in coordination with the disk on the other side of the pulley assembly.
The disclosures of the above identified patents to Emerson L. Kumm include control systems for effectively establishing the mutual angular relationship between the inner and outer disk members of each pulley assembly in the continuously variable transmission system. The mechanisms taught by the prior art cited above for establishing the mutual angular relationship between the inner and outer disk members has been relatively complex and expensive to manufacture. The present invention is directed to a simple mechanical mechanism for variably changing the relationship between the inner and outer disk members by means of axially directed forces.